1. Field of the Invention
The present invention relates to a positive temperature coefficient (PTC) polymer blend composition and circuit protection device including a PTC element made from the positive temperature coefficient (PTC) polymer blend composition.
2. Description of the Related Art
In recent years, positive temperature coefficient (PTC) polymer materials have been wildly applied to automatic limiting heating cables, over-current protection device e.g. thermistors, touch-control elements, and the like. Due to the extensive development, application and dissemination of electronic products, such as computers and peripheral equipments thereof, cellular phones, secondary rechargeable batteries, network interface boards/machines, modems and electronic facilities and so on, the need for over-current protection devices has remarkably increased. Particularly, the trend for present electronic products is to be light, delicate and accurate, and the characteristics of the over-current protection device made from PTC polymer materials are adapted to meet this trend.
PTC polymer materials primarily are prepared by the addition of conductive additives, such as carbon black and metal powders, to polymer materials. They are generally characterized by an increase in resistance as the operating temperature rises. In particular, when the operating temperature rises around and above the melting point of the PTC polymer material, the resistance increases sharply and logarithmically because of the sharp volume expansion, thus achieving an almost insulated condition. Such a phenomenon is a physical change of the PTC polymer material and is reversible and recurrent and thus, the PTC polymer material is suitable for application to a circuit protection device, such as a thermistor.
PTC polymer materials and circuit protection devices made therefrom have been disclosed in the following patents: U.S. Pat. Nos. 4,237,441, 4,304,987, 4,318,881, 4,226,633, 4,534,889, 4,560,498, 4,845,838, 5,227,940, 5,580,493, 5,747,147, 5,801,612, 3,351,882, 4,689,475, 4,800,253, 5,874,885, 5,940,958, 5,864,280, 5,800,668, and 6,059,997. These prior patents are incorporated herein for reference.
In a conventional circuit protection device made from the PTC polymer material, the PTC polymer material is used for forming a base. Metal foils are laminated or coated on the upper and lower sides of the base to act as electrodes. A conductive wire carrier or conductive sheet is connected to the outer side of the metal foils for enhanced connectivity.
Generally, the polymer materials used in the prior circuit protection devices include polyolefin, such as polyethylene and polypropylene, copolymers of polyolefin and derivatives thereof, such as EVA, EBA, EAA, EMAA, and EMA, and the mixture of polyolefin and copolymers of polyolefin and derivatives thereof. However, there are some defects in the application of the prior polymer materials. For instance, the adhesion of polyolefin to the metal foil electrodes is very poor. The polyolefin adhered to the electrodes is of poor processability and easily peels off and the contact resistance between the polyolefin and the surface of the electrodes is very high. Although the adhesion to the electrodes can be improved by the use of the copolymer of the polyolefin and derivatives thereof, the crystallinity of the copolymer of the polyolefin and derivatives thereof is relatively low and thus, the volume resistivity of the copolymer of the polyolefin and derivatives thereof rises accordingly. In other words, if a low volume resistivity is to be maintained, the amount of the conductive additives contained in the copolymer of the polyolefin and derivatives thereof has to be increased, but the physical properties of the thus formed copolymer composition will become relatively poor. Furthermore, on the basis of trip current being a function of heat transfer which is in terms of a function of melting point of polymer and polymer blends, since the melting point of the copolymer of the polyolefin and derivatives thereof (about 60.degree. C. to 100.degree. C.) is lower than that of polyolefin (about 125.degree. C. to 135.degree. C.) by 35.degree. C. to 75.degree. C., the trip current of the circuit protection device made from such a copolymer will accordingly decrease. In addition, although the adhesion of the mixture of polyolefin and the copolymer of polyolefin and derivatives to the electrodes is improved relative to that of polyolefin, such a mixture also undesirably includes some defects of the copolymer of polyolefin and derivatives thereof.
In view of the above defects, improvements by the use of metal foil electrodes of particular specifications to advance the adhesion property and the processability and to decrease the surface contact resistance have been disclosed in the prior art. U.S. Pat. No. 3,351,882 (Kohler et al.) discloses the use of electrodes of meshed construction to improve the adhesion of polymer materials to electrodes. However, in the disclosed device, the contact resistance between polymer materials and the surface of electrodes is high, and the distribution of current/voltage is uneven.
JP Kokai No. 5-10952 discloses the use of electrodes of a porous metal material having a three-dimensional network structure. However, such electrodes result in high resistance because of the difficulty in connecting a wire carrier.
U.S. Pat. Nos. 4,689,475 and 4,800,253 (Kleiner et al.) disclose a metal foil electrode having a chemically or mechanically micro-roughened surface. However, the roughened process increases the procedure complexity and cost of manufacture.
U.S. Pat. No. 5,874,885 discloses the use of two-layer metal foils including a base comprised of a first metal and protrusions on the base and comprised of a second metal so as to provide surface-roughened metal foil electrodes. Similarly, the roughened process increases the procedure complexity and cost of manufacture.
U.S. Pat. Nos. 5,955,936 and 5,940,958 disclose the use of electrodes characterized by a plurality of voids and made of foam. However, the production of such electrodes increases the procedure complexity and cost of manufacture.
U.S. Pat. Nos. 5,864,280, 5,880,668 and 6,059,997 disclose a crystalline PTC polymer composition, comprising a modified polyolefin and a conductive particulate filler, wherein the modified polyolefin is grafted to the conductive particulate filler. The modified polyolefin is a carboxylic acid-grafted polyolefin. The graft reaction between the modified polyolefin and the conductive particulate filler is carried out at a temperature of 240.degree. C. so as to allow the esterification between the carboxyl group of the modified polyolefin and the conductive particulate. The resultant circuit protection device made from the crystalline PTC polymer composition has good resistance stability. However, the graft reaction between the modified polyolefin and the conductive particulate filler has to be conducted at a temperature 240.degree. C., in close proximity to the critical operating temperature of the used grafted-polyolefin, and thus, the adverse effect, i.e. the decomposition of the carboxylic acid-grafted polyolefin into the carboxylic acid and the polyolefin, is likely to happen. Moreover, the graft temperature of the modified polyolefin and the conductive particulate filler, 240.degree. C., is higher than the common operating temperature, 180.degree. C. to 200.degree. C., by up to 60.degree. C. and increases the difficulty of the compounding process and graft reaction process of the composition.
In addition, the graft level and the uniformity of the modified polyolefin and the conductive particulate filler are difficult to be determined and thus, result in the uncertain yield of the resultant composition and the circuit protection device made therefrom.
There is thus a need for a PTC polymer composition which can be easily produced, and which has a good adhesion to electrodes and a good PTC behavior.